Substantial activity from the novel coronavirus combined with a major flu season this fall could place a tremendous burden on the healthcare system, a top U.S. health official said on Thursday.

"We're just not prepared," said Robert Redfield, director of the Centers for Disease Control and Prevention (CDC), in prepared testimony before a House of Representatives committee.

He also implied that as devastating as the coronavirus has been, a worse pandemic is bound to come. "You think we weren't prepared for this?" he mused. "Wait until we have a real global threat for our health security."

Concerns that a second wave of the virus could also spill over into flu season in the fall have heightened fears of the potential pressure on the nation's healthcare system.

The CDC is seeking emergency use authorization for a test to detect and differentiate flu from COVID-19, according to Redfield.

He said in the prepared testimony the CDC is working with drug makers to maximize the availability of influenza vaccines, and with healthcare providers "to develop contingency plans so that people can be vaccinated in a safe environment."

Major flu vaccine makers include British-based GlaxoSmithKline, France's Sanofi and Australia's CSL.

U.S. pharmacy chains have been preparing a big push for flu vaccinations in October, hoping to prevent tens of thousands of serious cases that could flood hospitals along with new COVID-19 cases.

Redfield also said the World Health Organization (WHO) continues to be a "close colleague" in public health efforts.

President Donald Trump said on Friday the United States will end its relationship with the WHO over the body's handling of the coronavirus pandemic.

During Redfield's appearance, Democrats criticized the Trump administration's response to the outbreak, which has led to more than 107,000 U.S. deaths.

"I have such admiration for the work that you and the CDC do, but if you and the CDC are driving this bus, you're taking us in a dangerous direction," said Rosa DeLauro, a Democratic congresswoman from Connecticut.

Asked about the delay in widely available coronavirus testing long into the U.S. outbreak, Redfield pointed to corporate America.

"It took unfortunately weeks and weeks and weeks before the private sector stepped up and developed what we now have," he told the panel, noting that more than 17 million coronavirus tests have now been done in the United States.

(With input from Reuters)